“Young women can cut their risk of developing breast cancer substantially by taking regular exercise”, The Daily Telegraph reported today. It says a US study has found that by doing regular exercise...

“Young women can cut their risk of developing breast cancer substantially by taking regular exercise”, The Daily Telegraph reported today. It says a US study has found that by doing regular exercise before they reach the menopause, women can reduce their risk of developing breast cancer by up to 23%. The study found women who exercised regularly between the ages of 12 and 22 benefited the most.

There are many risk factors for breast cancer and modifying one thing in particular will not eliminate all risk. Although large, this study has several limitations that are mainly to do with the methods used to collect the data for the study. The women in the study were expected to recall the levels of physical activity up to 35 years ago. In addition, several factors that may have had a greater effect on the risk of breast cancer than their exercise history were not balanced between the women who exercised more, or less, frequently. Furthermore, there is the possibility that women who take regular exercise are also more likely to have other factors that protect them from the disease, such as being less prone to being overweight or eating a healthier diet.

However, despite the study’s limitations, its basic message is a good one. A healthy and active lifestyle should always be aimed for and maintaining such a lifestyle is known to reduce the risk of cancer and other disease.

Where did the story come from?

Sonia S. Maruti and colleagues from the Brigham and Women’s Hospital and Harvard Medical School; the Harvard School of Public Health; the University of Washington, and Fred Hutchinson Cancer Research Center, Seattle, carried out the research. The study was funded by the National Cancer Institute, National Institutes of Health, and the American Cancer Society.

The study was published in the peer-reviewed medical journal: Journal of the National Cancer Institute.

What kind of scientific study was this?

In this cohort study, the researchers aimed to investigate whether physical activity lowers the risk of premenopausal breast cancer. The researchers also wanted to identify, if possible, the age that exercise was most critical in reducing risk.

For this study, the researchers used data from the Nurses’ Health Study II, a large ongoing group study that began in 1989. The study enrolled 116,608 female nurses between the ages 25-42 by questionnaire. The women have since been sent follow-up questionnaires every two years in order to keep up-to-date information on their lifestyle and report any newly diagnosed medical conditions.

This particular study followed the women over six years, starting in 1997. By this time, the women were 33-51 years of age. Only women who had reported doing physical activity in both adolescence and adulthood were included. The researchers also excluded women with a diagnosis of any cancer prior to 1997, who had non-invasive breast cancer, or were postmenopausal. This left a study group of 64,777 women.

The women gave information on how many hours per week they spent walking, doing moderate leisure activities, such as hiking or casual cycling, and strenuous activities such as running or lap swimming. This was done for five age periods (ages 12-13; 14-17; 18-22; 23-29; 30-34). The researchers also looked at occupational activity during the latter age periods, (i.e. if the women were mostly sitting or doing some degree of manual labour).

The hours per week that the women did the activities were assigned metabolic equivalent (MET) values, a measure of the effort required to do that activity, and added up to give a total for the year. The women were then analysed in each of the time brackets and an average leisure-time activity was worked out for different age periods and across the women’s lifetime.

The women’s self-reports of breast cancer in the questionnaire were verified by an independent doctor, who looked at the woman’s medical records and pathology reports. Other details of possible factors that can affect risk of breast cancer, such as the age when the women’s periods started, contraceptive use and reproductive history, family history of breast cancer and smoking and weight, were obtained from questions at various time points during the study.

What were the results of the study?

There were 550 cases of invasive premenopausal breast cancer during the follow up period. There were many differences between the women that had lower lifetime MET hours per week compared to those with higher lifetime MET hours per week. This included factors such as age, weight, alcohol, smoking, animal fat consumption, multivitamin use and whether they had children.

The amount of walking, moderate and strenuous activity has no effect on the risk of breast cancer. The only significant reduction in the risk of premenopausal breast cancer was seen in women with a total activity level of between 39 and 53.9 MET hours per week (26% decreased risk). Exercise levels of more or less than this did not have an effect on breast cancer risk.

The researchers also looked separately at the effect of different activity levels during each of three time periods (age 12-22; 23-34; 35 or over). They found that only women who did 72 or more MET hours per week between the ages of 12 and 22 had a decreased risk of premenopausal breast cancer.

What interpretations did the researchers draw from these results?

The researchers say that leisure time activity was associated with a reduced risk of premenopausal breast cancer. They conclude that women engaging in high amounts of physical activity during both adolescence and adulthood may derive most benefit.

What does the NHS Knowledge Service make of this study?

Although this was a large group study, there are several limitations. Some points to consider when interpreting these findings are:

All physical activities were self-reported by questionnaire. The regularity of the women’s exercise type and its frequency is unlikely to remain consistent over time and the average weekly amount is likely to be an estimate only. The metabolic effort that the researchers attached to activities they thought to be strenuous or moderate may also not be representative for all women. For instance, the effort that one woman puts into half an hour of swimming or walking may not be the same as for someone else.

Importantly, the women were asked to recall their activity levels from as far back as 12 years of age. It is unlikely whether these recollections would be accurate in all cases.

The reduction in risk that was seen for certain amounts of metabolic activity is going to be very difficult for most people to interpret or put into practise, i.e. how to do between 39 and 53.9 MET hours per week. Although the researchers do suggest this is about 3.25 hours per week of running or 13 per week of walking.

Although many possible confounding factors were taken into account in the risk analyses, these would have again been dependent on the women’s memory, and may not be entirely accurate across women (e.g. age periods started or duration of oral contraceptive use). In addition, other possible risk factors, such as dietary intake were not considered.

There are many factors associated with breast cancer risk and the women with differing levels of physical activity differed in several ways. Modifying one factor in particular will not eliminate all risk.

These findings only apply to premenopausal white women. The fact that they were all registered nurses may also mean that they were from a particular socioeconomic category.

The headline that exercise helps the young ‘beat’ breast cancer should not be misinterpreted to mean that if a woman has breast cancer she will be more likely to recover from the disease if she exercises.

However, despite the limitations of the study, a healthy and active lifestyle should always be aimed for. It is also known that maintaining such a lifestyle is associated with a reduced risk of cancer and other disease.

Sir Muir Gray adds...

Yet more evidence about the benefits of being active, the simplest and best preventive measure against cancer for non-smokers. People who smoke should stop and be more active.